Typically, the optical systems used in the context of fabricating microelectronic devices such as semiconductor devices include a plurality of optical element units including optical elements, such as lenses and mirrors etc., arranged in the exposure light path of the optical system. Those optical elements usually cooperate in an exposure process to transfer an image of a pattern formed on a mask, reticle or the like onto a substrate such as a wafer. The optical elements are usually combined in one or more functionally distinct optical element groups. These distinct optical element groups may be held by distinct optical exposure units. In particular with mainly refractive systems, such optical exposure units are often built from a stack of optical element modules holding one or more optical elements. These optical element modules usually include an external generally ring shaped support device supporting one or more optical element holders each, in turn, holding an optical element.
Due to the ongoing miniaturization of semiconductor devices there is, however, a permanent need for enhanced resolution of the optical systems used for fabricating those semiconductor devices. This need for enhanced resolution obviously pushes the need for an increased numerical aperture (NA) and increased imaging accuracy of the optical system.
One approach to achieve enhanced resolution is to reduce the wavelength of the light used in the exposure process. In the recent years, approaches have been taken using light in the extreme ultraviolet (EUV) range, typically using wavelengths ranging from 5 nm to 20 nm, in most cases about 13 nm. In this EUV range it is not possible to use common refractive optics any more. This is due to the fact that, in this EUV range, the materials commonly used for refractive optical elements show a degree of absorption that is too high for obtaining high quality exposure results. Thus, in the EUV range, reflective systems including reflective elements such as mirrors or the like are used in the exposure process to transfer the image of the pattern formed on the mask onto the substrate, e.g. the wafer.
The transition to the use of high numerical aperture (e.g. NA>0.4 to 0.5) reflective systems in the EUV range leads to considerable challenges with respect to the design of the optical imaging arrangement.
One of the desired accuracy properties is the accuracy of the position of the image on the substrate, which is also referred to as the line of sight (LoS) accuracy. The line of sight accuracy typically scales to approximately the inverse of the numerical aperture. Hence, the line of sight accuracy is a factor of 1.4 smaller for an optical imaging arrangement with a numerical aperture NA=0.45 than that of an optical imaging arrangement with a numerical aperture of NA=0.33. Typically, the line of sight accuracy ranges below 0.5 nm for a numerical aperture of NA=0.45. If double patterning is also to be allowed for in the exposure process, then the accuracy would typically have to be reduced by a further factor of 1.4. Hence, in this case, the line of sight accuracy would range even below 0.3 nm.
Among others, the above can lead to very strict desired properties with respect to the relative position between the components participating in the exposure process as well as the deformation of the individual components. Furthermore, to reliably obtain high-quality semiconductor devices it is not only desirable to provide an optical system showing a high degree of imaging accuracy. It is also desirable to maintain such a high degree of accuracy throughout the entire exposure process and over the lifetime of the system. As a consequence, the optical imaging arrangement components, i.e. the mask, the optical elements and the wafer, for example, cooperating in the exposure process are supported in a well-defined manner in order to maintain a predetermined spatial relationship between the optical imaging arrangement components and to provide minimum undesired deformation caused by parasitic residual loads as well to provide a high quality exposure process.
To maintain the predetermined spatial relationship between the optical imaging arrangement components throughout the entire exposure process, even under the influence of vibrations introduced, among others, via the ground structure supporting the arrangement and/or via internal sources of vibration disturbances, such as accelerated masses (e.g. moving components, turbulent fluid streams, etc.), as well as the under the influence of thermally induced position alterations, it is desirable to at least intermittently capture the spatial relationship between certain components of the optical imaging arrangement and to actively adjust the position and/or orientation of at least one of the components of the optical imaging arrangement as a function of the result of this capturing process.
Generally, two different concepts are known for achieving such active adjustment. One is the so-called active support using contactless force actuators, e.g. so-called Lorentz actuators, generating a defined support force as a function of a given input signal. While these force actuators have clear advantages in terms of their dynamic properties as well as the comparatively low parasitic residual loads on the optical components, they involve a fairly complex support structure as well as sophisticated and expensive control layout. Moreover, they are susceptible to aging effects and they constantly have to be provided with power. As a result, they generate heat which has to be removed from the system in order to avoid thermally induced deformation of the optical components and, ultimately, deterioration of the imaging quality obtained.
Another approach to achieve such active adjustment is the so-called semi-active support using displacement actuators, such as e.g. piezoelectric actuators, generating a defined displacement as a function of a given input signal. Such semi-active supports have the advantage that they are comparatively cheap to implement, involve less sophisticated control, show very little aging effects and generate way less heat compared to the above active concepts using force actuators.
Typically, such displacement actuators are used in so-called hexapod arrangements, usually formed by three bipods distributed at the circumference of the optical component to be supported. These hexapod arrangements, thanks to a sufficiently well decoupling linkage to the optical component and the support structure, respectively, provide statically determined support (often also referred to as isostatic support) of the supported the optical component with comparatively low residual parasitic loads introduced into the optical component.
As a result of the soft decoupling linkage, however, the rigidity of the support system also is comparatively low in certain degrees of freedom, typically in the translational degrees of freedom lying in the support plane defined by the hexapod. This is in many cases undesirable under dynamic aspects.